1. Field of the Invention
This invention relates to optical beam splitter prisms and, in particular, to optical beam splitter prisms having orientation insensitivity. Accordingly, it is a general object of this invention to provide new and improved optical beam splitter prisms of such character.
2. General Background
An optical beam splitter is based upon the separation of an incident wave, at a partially reflecting interface between two transparent media, into a reflected wave and a transmitted wave. The interface may consist of a thin metallic layer or it may consist of a multilayer thin film stack of dielectric coatings or it may consist of a combination thereof, and may be designed to reflect part of the beam intensity regardless of its polarization state or wavelength, or so as to selectively reflect one polarization state, or a band of wavelengths, and to transmit the remaining light energy. In both cases, the interface is usually planar to avoid defocusing an applied beam upon reflection. A glass prism assembly usually supports the interface layers and provides for the light beam entrances and exits. Such a cube type beam splitter of the prior art is depicted in FIG. lA.
An incident optical beam 11 enters a glass beam splitter 12, as depicted in FIG. lA, and passes through a first 45.degree. prism half 13 to an interface 14. Part of the beam intensity is reflected and leaves the prism half 13 as a first output beam 16, while the remaining intensity is transmitted through a second 45.degree. prism half 17 and leaves as a second output beam 18. The beam 16 is nominally ninety degrees in direction from that of the beam 18, but if, as depicted in FIG. IB, the prism 12 is rotationally misaligned, the output beam 16 is changed in direction relative to the input beam 11. The output beam 18, having been refracted twice by the parallel facets of the beam splitter 12, is laterally displaced by a small amount but is still parallel to the input beam 11.
Beam splitter prism devices are useful in fiber optic systems. In these applications, the light from an optical fiber is focused into a parallel beam by a small lens, passed through a prism, and then refocused back into another fiber. The small lenses, affixed to the fiber with their focal point at the end of the fiber core, are known as expanded beam lens connectors and serve as convenient interfaces between light in a fiber and light in a parallel beam. But, for complete coupling into the fiber, the input parallel beam should have a high degree of angular accuracy with respect to the lens axis. Due to the expanded cross-sectional area of the parallel beam, lateral positioning is of secondary importance.
The shortcomings of the prior art beam splitter shown in FIGS. 1A and 1B are apparent. If optical beams 11, 16, and 18 are coupled into optical fiber with expanded beam lens connectors, the angular accuracy and coupling efficiency of the beam 16 is critically dependent upon the orientation of the beam splitter cube 12. The beam 18 which remains parallel, but with a small lateral displacement, couples much more completely with its fiber.